Apes to hominids

Primates are an order of the class Mammalia. The Primate order was initially divided into two suborders—Prosimii (lower primates), which includes lemurs, lorises, and tarsiers, and Anthropoidea (higher primates), including monkeys, apes, and humans. However, tarsiers were later identified as close relatives of anthropoids, leading to the revision of primate classification suborders—Strepsirrhini (lemurs and lorises) and Haplorrhini (tarsiers, monkeys, and apes, including humans). Anthropoidea was divided further into infraorders—Platyrrhini (flat-nosed New World monkeys, not ancestral to man) and Catarrhini (down-nosed Old World monkeys, apes, and man). The infraorder Catarrhini includes two superfamilies—Cercopithecoidea (Old World monkeys) and Hominoidea (apes and man). Within the Hominoidea, there are three families—Hylobatidae (lesser apes), Pongidae (great apes), and Hominidae (man). As this classification suggests, it is now taken for granted that human ancestry—if it could be traced satisfactorily—would include forms that, on other genealogies, gave rise to lower and higher primates, Old World monkeys, and a series of now-extinct creatures that were ancestral to certain apes as well.

The lower primates (prosimians) first appeared about seventy million years ago. They still exist (as lemurs, lorises, and tarsiers) but have been declining for the last thirty million years, probably because of unsuccessful competition with their own descendants, the monkeys. Prosimians have five digits on each limb, but the digits have claws rather than nails, and the limbs are entirely quadrupedal. Prosimians also lack binocular vision, but they do have dentition anticipating the molar development of the higher primates.

The Higher Primate Fossil Record

The earliest evidence of any kind of higher primate—some tiny pieces of jaw found in Burma—dates from the Eocene epoch, about sixty-five million years ago. Two creatures named Amphipithecus (“near ape”) and Pondaungia (“found in the Pondaung Hills”) have been proposed, each being a very primitive monkey or ape, but the evidence thus far is too sparse to ally these forms with any possible descendants. Some two million years later, in the Fayum Depression of Egypt (then a lush forest), Apidium and Parapithecus (“past ape”) existed. Known only from jaws and teeth, they are the oldest known Old World monkeys presently recognized. Their dental pattern (arrangement of teeth), however, is the same as that of Amphipithecus. Other teeth from the Fayum, perhaps thirty-five to thirty million years old, have a different cusp pattern, more like an ape’s (and a human’s) than a monkey’s. Possibly, then, Propliopithecus (“before more recent ape”) is the earliest evidence of an ape line distinct from the monkey line.

The oldest apelike animal about which scientists know enough to regard it as a probable human ancestor is Aegyptopithecus (“Egyptian ape”), also found in the Fayum, in Oligocene deposits about thirty-two million years old. In addition to jaws and teeth, an almost complete skull and some postcranial bones (meaning those below the skull) have been recovered. Since the Fayum at that time consisted of dense tropical rainforest with little open space, Aegyptopithecus is assumed to have been an arboreal quadruped. (In 1871, before Aegyptopithecus was known, Charles Darwin had predicted that such a human ancestor existed.)

Proconsul (“before Consul,” Consul being a chimpanzee in the London Zoo in 1933) and Dryopithecus (“forest ape”) were either closely related to each other or identical. Proconsul appeared in Africa at the start of the Middle Miocene (about twenty million years ago) and was contemporary with Dryopithecus in Europe and Asia about fourteen million years ago. The relatively abundant fossils of these forms have been classified by some researchers into three species, together forming an extinct subfamily, the Dryopithecinae. One species in particular, Dryopithecus major, regularly left its remains on what were then the forested slopes of volcanoes; males grew significantly larger than females (a situation known as sexual dimorphism). In both of these respects, Dryopithecus resembled the modern gorilla, to which it may be ancestral. Like the gorilla, Dryopithecus major probably walked on its knuckles. In size, it was somewhat larger than a chimpanzee. The other two species, Dryopithecus nyanzae and Dryopithecus africanus, were smaller than Dryopithecus major and more like the chimpanzee. Outside Africa, Dryopithecus has been found from Spain to China.

The Ancestors of Modern Apes and Humans

Limnopithecus (“lake ape”), found in deposits in Kenya and Uganda about twenty-three to fourteen million years ago, is thought to be an earlier form of Pliopithecus (“more recent ape”). Its gibbonlike skulls, jaws, and teeth are plentiful in European sediments of Middle Miocene to Early Pliocene age—sixteen to ten million years ago. For some researchers, these two forms constitute a separate subfamily, the Pliopithecinae, which they consider to be part of the family Hylobatidae (lesser apes). In some respects, they resembled the modern gibbon, but other aspects of their anatomy were quite different. For example, Pliopithecus possessed seven lumbar vertebrae, whereas gibbons (and humans) have only five. It seems to have been primarily arboreal, swinging from branch to branch. Pliopithecus has been known since 1837 (in France), and since then, some almost-complete skeletons have been recovered. Sivapithecus (“Siva’s ape,” Siva being a Hindu deity), found in India and later in East Africa, is a closely related form, Miocene in age. Both the dryopithecines and the pliopithecines are often regarded as the ancestors of modern apes. The earliest known ancestor of the modern-day gibbon, Kapi ramnagarensis, was discovered in India in the early 2020s. This thirteen-million-year-old discovery provided scientists with clues about the gibbon's migration from Africa to Asia and filled a major fossil record gap in ape history. Around the same time, Yuanmoupithecus xiaoyuan was discovered in China's Yunnan province, dated to the late Miocene period between seven and eight million years ago, making it the earliest known hylobatid.

Ramapithecus (“Rama’s ape,” Rama being another Hindu deity), found originally as a jaw fragment in India, is remarkable for its human-looking teeth. Some researchers regard it as the earliest member of the hominid line and, therefore, ancestral to humans. Others, however, relate Ramapithecus and Sivapithecus to modern orangutans, seeing no direct connection to man. Though Ramapithecus has been recovered from Late Miocene deposits in Africa and Indian and Early Pliocene ones in India (about fourteen to ten million years ago), only teeth and jaws have been found. As a result, many opinions regarding Ramapithecus are highly conjectural. The most striking feature of this genus, for example, is the greatly reduced size of its canine teeth, as compared with those of earlier (as well as modern) apes. Presumably, this indicates a changed diet of some sort. However, primates also use their teeth for nondietary purposes, including weaponry and display. It has, therefore, been suggested that the reduced tooth size of Ramapithecus might indicate it had begun to use other tools or weapons; if so, none has ever been found. Another conjecture has been that climatic change brought the primates down from the trees. Once on the ground, Ramapithecus then developed a hunter-gatherer style of sustenance that eventually included the formation of family units (male-female bonds), tool making, and a rudimentary form of language—the beginnings of culture. All that is really known about Ramapithecus is what can be observed from a smattering of its bones.

Finally, there was Gigantopithecus (“giant ape”), a huge simian with protohuman teeth (clearly not ancestral to man, however) that outlasted Dryopithecus, Ramapithecus, and Sivapithecus to survive in Asia for almost nine million years. The largest primate that ever evolved (exactly how large is not known), it was alive in China as recently as a million years ago. Known for its immense molars, Gigantopithecus was apparently the only successful ground-living savanna ape. It probably competed with early hominid forms and may have been exterminated by them.

In broad outline, these are fossil apes. Since much of the evidence (all of it, in several cases) consists of teeth and jawbones, it is not surprising that conjecture has played a very active part in attempts to associate this evidence with the evolution of the hominids. Before 1980, there was widespread consensus among experts with regard to an evolutionary main line extending at least from Aegyptopithecus through Dryopithecus (or Proconsul) and Sivapithecus to Ramapithecus, the latter being regarded as the first hominid. However, portions of two Sivapithecus faces, recovered from Turkey in 1980 and Pakistan in 1982, impressed researchers with their orangutan-like characteristics. Since firm ties between Sivapithecus and Ramapithecus had already been established, it began to seem that the entire lineage pointed toward the orangs rather than toward man. Another problem is that formerly accepted dating has come into question for such important branchings of the lineage as those that separated monkeys from apes and apes from man. New genetic studies having nothing to do with either fossils or stratigraphy have presented compelling (but controversial) arguments to the effect that these branchings occurred much later than hitherto believed. A third, even more, serious problem is that there is little pertinent fossil evidence regarding the development of simian primates into humans for a period beginning about twelve million years ago and lasting until the appearance of the australopithecines about four million years ago. While anthropologists and biologists continue to learn more about the ancestry of modern simians, it is not the case that a reliable lineage (or timetable) leading from other primates to humankind has been established.

The Problems of Theorizing from Fossils

The study of fossil apes is a specialization within the broader field of vertebrate paleontology, or the study of fossil bones. Like all paleontologists, paleoprimatologists are necessarily concerned with fossils and their stratigraphic occurrence. Because primates still exist, however, it is also important to study the behavior of living examples. Since behavior reflects environmental conditions, it is further necessary to reconstruct the climate, flora, and fauna of the region and time in which the fossils were found.

No complete fossil ape has ever been found. Any understanding of what they may have looked like is, therefore, conjectural—an extrapolation from what has been recovered to what has not. Skulls are undoubtedly the most desirable evidence, but they are not the most durable of fossils. Teeth, which constitute the hardest parts of the primate body, are preserved more often than any other part. Some kinds of fossil apes are known either exclusively or primarily from their teeth and jaws. Ape teeth differ from human teeth in two significant respects: They are generally larger (the canines especially), and the cusp patterns on their molars differ. The arrangement of teeth in an ape’s jaw, moreover, is angular, like a V. In humans, the arrangement is rounder, like a U. Inevitably, whenever jawbones or molar teeth are found, an attempt is made to place them somewhere on a continuum that runs between the purely simian (ape) and the purely human. This procedure distinguishes primitive apes from primitive humans and one kind of fossil ape from another but also gives rise to inevitable conjecture as to possible anticipations of the human line.

A major difficulty with evolutionary sequences based solely upon dental evidence is that the head and body of a given species have not necessarily evolved at the same rate. One may be surprisingly apelike, the other somewhat human. Even more specifically, the fact that jaws are changing does not necessarily mean that crania (or any other specific body parts) are also changing. On the whole, scientists do not yet understand the evolution of primate anatomy well enough to interpret present evidence or reconstruct missing parts with much reliability. In the absence of factual evidence, the form taken by prevailing reconstructions at any given time may owe as much to professional politics as to objective knowledge. The controversy regarding Ramapithecus, which (on the basis of facial bones) moved that genus from a central position at the base of the human lineage to a similar position on a separate orangutan genealogy, has been a valuable lesson in the folly of premature commitment.

Ancient Fossils and Modern Primates

Most paleoprimatologists are, to some extent, modern-day primatologists who are experts in the comparative anatomy of all higher primates and as knowledgeable as possible regarding their behaviors and environments. It is assumed that a changing environment (one becoming increasingly arid, for example) requires behavioral modifications and that these modifications will then create selection pressures favoring some types of anatomical variation over others; thus, one species will eventually change into (or be replaced by) another. The process is not well understood, but the primates—being so intensely studied—are often regarded as test cases for competing evolutionary theories. Some researchers stress evidence to the effect that species are always changing; others believe that species are created precipitously and then tend to endure relatively unchanged until they are abruptly superseded. All that is known is that the ancient apes generally conform to a partially ascertainable progression from hypothetical earlier forms to modern-day primates.

The Importance of Fossil Apes

Fossil apes are important for many reasons. They are an essential group in their own right, but also critical in the development of mammals and primates. They are thought to be ancestral to humankind and, therefore, uniquely important among all nonhuman fossil genera.

One of the unique characteristics of humans is the ability to create, preserve, and transmit knowledge. Homo sapiens have learned to value learning, hoard, and increase knowledge in the realization that it fortifies and enhances their existence. Humans attempt to know and understand all present-day forms of life. To do so, however, it must also be known how these forms came into being through time. Biology, then, is inherently evolutionary and does not sharply distinguish between plants and animals of the past and those of the present.

Insofar as an understanding of life itself is the goal of biological studies, no single form of life is inherently more important than any other. From this point of view, one would say that fossils and living apes are studied for the same reason that algae, sponges, or nematodes are. Many biologists maintain this view. Many others, however, believe mammals—and especially primates—to be a “higher” form of life, anatomically more complex than sponges (though not necessarily more nearly perfect) and certainly capable of more complex behaviors. No mere study of anatomy, this viewpoint suggests, can sufficiently explain a primate.

The outstanding characteristic of all primates is their intelligence. However, one can find surprising levels of intelligence in other animals. Among invertebrates, such cephalopods as the squid and the octopus have highly developed nervous systems, senses, and brains that are normally associated with mammals. Together with some birds and social insects, all mammals are capable of surprisingly complex behavior. Nevertheless, the higher primates constitute an intellectual elite even among the mammals. Impressive as gorillas and chimpanzees can be in this respect, it is apparent that the human mind has a capacity well beyond theirs. Scientists assert that the brains of extinct apes are ancestral not only to modern apes but also to humans.

Scientists are fortunate in the number of fossil ape skulls that have been found, for they make the increasing mental capacity of the higher primates easy to establish. Limb bones and other less durable parts of the skeleton are much rarer. When available, they indicate the relative lengths of arms and legs; the nature of the shoulder (a key to arboreal existence); the relation of pelvis and femur (a key to posture); and the shapes, capabilities, and functions of hands and feet. Without such evidence, scientists have only conjectures based on the presumed place of the genus in question within a supposed evolutionary sequence. When proposed sequences differ, though they are derived from the same sparse evidence, conflicting suppositions about the evolutionary sequence are at work.

Principal Terms

Apes: Large, tailless, semierect anthropoid primates, including chimpanzees, gorillas, gibbons, orangutans, and their direct ancestors—but excluding man and his direct ancestors

Australopithecines: Nonhuman hominids commonly regarded as ancestral to man

Dryopithecines: Extinct Miocene-Pliocene apes; their evolutionary significance is unclear

Hominid: An anthropoid primate of the family Hominidae, including the genera Homo and Australopithecus

Human: A hominid of the genus Homo, whether Homo sapiens sapiens (to which all varieties of modern man belong), earlier forms of Homo sapiens, or such presumably related types as Homo erectus

Primates: Placental mammals, primarily arboreal, whether anthropoid (humans, apes, and monkeys) or prosimian (lemurs, lorises, and tarsiers)

Stratigraphy: In geology, a sequence of sedimentary or volcanic layers, or the study of them—indispensable for dating specimens

Bibliography

Berger, Lee. In the Footsteps of Eve: The Mystery of Human Origins. Adventure Press/National Geographic Society, 2000.

Begun, David R. The Real Planet of the Apes: A New Story of Human Origins. Princeton University Press, 2018.

Campbell, Bernard. Humankind Emerging. 8th ed. Struik Nature, Cape Town, 2019.

Conroy, Glenn C. Primate Evolution. W. W. Norton, 1990.

Jordan, Paul. Neanderthal: Neanderthal Man and the Story of Human Origins. The History Press, 2013.

Lewin, Roger. Bones of Contention: Controversies in the Search for Human Origins. 2nd ed. Simon & Schuster, 2001.

Martin, R. D. Primate Origins and Evolution: A Phylogenetic Reconstruction. Chapman and Hall, 1990.

McRae, Ryan, and Briana Pobiner. "Fourteen Discoveries Made About Human Evolution in 2022." Smithsonian Magazine, 27 Dec. 2022, www.smithsonianmag.com/smithsonian-institution/fourteen-discoveries-made-about-human-evolution-in-2022-180981344. Accessed 20 Sept. 2024.

Szaly, Frederick S., and Eric Delson. Evolutionary History of the Primates. Academic Press, 1979.

Whitfield, John. Lost Animals: Extinct Endangered and Rediscovered Species. Smithsonian Books, 2020.

Xueping Ji, et al. "The Earliest Hylobatid from the Late Miocene of China." Journal of Human Evolution, vol. 171, 2022, doi.org/10.1016/j.jhevol.2022.103251.